This invention relates to a plastic container closure suitable for a container for a beverage, especially a container for a carbonated beverage.
A container formed from a plastic material, such as polyethylene terephthalate or glass, is widely used as a container for a beverage. Such a container includes a nearly cylindrical mouth-and-neck portion, and an external thread is formed on an outer peripheral surface of the mouth-and-neck portion. An upper end part of the mouth-and-neck portion, normally positioned above the external thread, has an annular top surface extending substantially horizontally, a cylindrical outer peripheral surface extending substantially vertically, and a cylindrical inner peripheral surface extending substantially vertically. The annular top surface and the cylindrical inner peripheral surface are connected together, substantially directly or via an annular boundary surface extending arcuately over a slight length in a sectional view. Similarly, an annular boundary surface, extending substantially arcuately over a considerable length in a sectional view, exists between the annular top surface and the cylindrical outer peripheral surface.
Recently, a plastic container closure formed from a plastic material, such as high density polyethylene or polypropylene, has found practical use as a container closure for sealing the mouth-and-neck portion of a container having the above-described shape. A typical example of such a plastic container closure has a circular top panel wall, and a cylindrical skirt wall extending downwardly from a peripheral edge of the top panel wall, as disclosed, for example, in Japanese Unexamined Patent Publication No. 73551/1983. An internal thread is formed on an inner peripheral surface of the skirt wall. An annular seal piece, which may extend downwardly obliquely in a radially inward direction, is formed in an outer peripheral edge portion of the inner surface of the top panel wall. An annular contact piece positioned radially inwardly of the annular seal piece, and an annular positioning piece extending downwardly at a position radially inward of the contact piece are also often formed in the outer peripheral edge portion of the inner surface of the top panel wall.
In mounting the container closure on the mouth-and-neck portion of the container to seal the mouth-and-neck portion, the container closure is capped over the mouth-and-neck portion, and turned in a closing direction to screw the internal thread of the container closure onto an external thread of the mouth-and-neck portion. During this motion, the annular positioning piece of the container closure is lowered along the inner peripheral surface of the upper end part of the mouth-and-neck portion. By this action, the container closure is aligned with the mouth-and-neck portion to match the center line of the container closure to the center line of the mouth-and-neck portion. When the internal thread of the container closure is screwed onto the external thread of the mouth-and-neck portion with a required rotating torque, the annular seal piece of the container closure is intimately contacted with the outer peripheral surface in the upper end part of the mouth-and-neck portion, whereby the mouth-and-neck portion is sealed. The contact piece of the container closure is brought into contact with the substantially horizontally extending top surface of the mouth-and-neck portion, whereby the vertical position of the container closure relative to the mouth-and-neck portion is restricted.
The conventional plastic container closure of the foregoing shape poses the following problem to be solved: Assume that a carbonated beverage containing a carbon dioxide gas is held in the container. When the ambient temperature reaches 40 degrees centigrade, for example, a considerably high gas pressure (e.g., a gas pressure of about 6 kg/cm2) is generated in the container. Such a gas pressure causes a so-called dome phenomenon in which the top panel wall of the container closure is deformed upward in a domic shape. As is well known among people skilled in the art, a container filled with a carbonated beverage and mounted with a container closure is usually accommodated in a carton (a box made of a folding boxboard). The cartons accommodating a required number of the containers are stored and transported in a stacked condition. During their storage and transportation, a considerable load (a load, for example, of about 10 kg) is imposed on the top panel wall of the container closure mounted on the mouth-and-neck portion of the container. According to experiments conducted by the inventors of the present invention, when a considerably great stacking load is imposed on the top panel wall of the container closure subject to the dome phenomenon, the seal on the mouth-and-neck portion by the container closure tends to be destroyed, resulting in leakage of the gas pressure from inside the container. To solve this problem, it is conceivable to form a sealing liner of a required shape on the inner surface of the top panel wall of the container closure, separately from the container closure body, by a well-known embossing method. By so doing, however, the manufacturing cost of the container closure is considerably increased.
A principal object of the present invention is to provide an improved plastic container closure which can be produced without an increase in the manufacturing cost, and which nevertheless, enables sealing of the mouth-and-neck portion by the container closure to be maintained fully reliably, even when a considerably great stacking load is imposed on the top panel wall of the container closure undergoing a dome phenomenon.
As a result of extensive studies and experiments, the inventors found the following facts, to their surprise: When the annular contact piece is changed to such a shape as to contact the annular boundary surface extending arcuately in the sectional view, rather than contacting the substantially horizontally extending top surface of the mouth-and-neck portion, sealing of the mouth-and-neck portion by the container closure can be maintained fully reliably, even if a considerably great stacking load is imposed on the top panel wall of the container closure when the above-mentioned dome phenomenon is caused to the top panel wall of the container closure. The reason why sealing of the mouth-and-neck portion can be maintained fully reliably by such a change applied to the annular contact piece of the container closure is not necessarily clear, but the inventors believe it to be as follows: Generally, when the dome phenomenon is caused to the top panel wall, the top panel wall is deformed upward in a region inward of the annular boundary surface of the mouth-and-neck portion. In other words, the top panel wall bulges upward more greatly in its region closer to its center, with the annular boundary surface of the mouth-and-neck portion as a fulcrum. If the annular contact piece of the container closure is in contact with the top surface inward of the annular boundary surface of the mouth-and-neck portion, the top panel wall is deformed, when a stacking load is imposed on the top panel wall undergoing the dome phenomenon. In accordance with this deformation, the annular contact piece is also deformed or moved. At the same time, the deformation of the top panel wall is transmitted to the annular seal piece. As a result, the annular seal piece is also deformed to destroy sealing of the mouth-and-neck portion. On the other hand, when the annular contact piece of the container closure is brought into contact with the annular boundary surface of the mouth-and-neck portion, namely, the fulcrum for the deformation of the top panel wall, the tendency toward the deformation or movement of the annular contact piece by the deformation of the top panel wall is considerably lessened. Moreover, the tendency to deformation of the annular seal piece due to the deformation of the top panel wall is considerably suppressed. Hence, the sealing of the mouth-and-neck portion is maintained.
According to a first aspect of the present invention, as a plastic container closure for attaining the aforementioned technical object, there is provided a plastic container closure which is to be applied to a container having an annular top surface extending substantially horizontally, a cylindrical outer peripheral surface extending substantially vertically, and an annular boundary surface extending from the top surface to the outer peripheral surface substantially arcuately in a sectional view, the top surface, the outer peripheral surface, and the boundary surface being formed in an upper end part of a mouth-and-neck portion of the container, the container closure comprising:
a circular top panel wall; and
a cylindrical skirt wall extending downwardly from a peripheral edge of the top panel wall, and wherein
an annular seal piece and an annular contact piece are integrally formed in an outer peripheral edge portion of an inner surface of the top panel wall, the annular seal piece extending downwardly obliquely in a radially inward direction and being brought into intimate contact With the outer peripheral surface of the mouth-and-neck portion of the container, whereby the annular seal piece is deflected radially outwardly, and the annular contact piece being brought into contact with the boundary surface of the mouth-and-neck portion.
In a preferred embodiment, the annular contact piece is composed of a bulgy portion which bulges downward in a radially inward direction and is compressed upon contact with the boundary surface of the mouth-and-neck portion of the container. The annular contact piece may be of such a shape as to have an inclined surface which extends obliquely upwardly from a base portion of the annular seal piece in a radially inward direction, and to be compressed upon contact with the boundary surface of the mouth-and-neck portion. The annular contact piece may also be composed of a protrusive portion which protrudes downwardly or radially inwardly, and is deflected upward upon contact with the boundary surface of the mouth-and-neck portion of the container. When the minimum inner diameter of the annular seal piece is designated as D1, and the outer diameter of the outer peripheral surface of the mouth-and-neck portion is designated as D2, it is preferred that D2=D1=0.30 to 1.00 mm. Preferably, a cylindrical inner peripheral surface extending substantially vertically is formed in the upper end part of the mouth-and-neck portion of the container, and an annular positioning piece, located radially inwardly of the contact piece and extending downward, is formed in the outer peripheral edge portion of the inner surface of the top panel wall. When the maximum outer diameter of the annular positioning piece is designated as D3, and the inner diameter of the inner peripheral surface of the mouth-and-neck portion of the container is designated as D4, it is preferred that D4xe2x88x92D3=0.07 to 0.16 mm.
The inventors further conducted extensive studies and experiments, and found the following facts: When an annular thin-walled region is formed radially inwardly of the annular seal piece in the top panel wall, deformation of the top panel wall (i.e., a dome-shaped deformation, or the dome-shaped deformation forcibly returned to the original state) is restricted mainly to a region inward of the annular thin-walled region, and deformation of the top panel wall in an area radially outward of the annular thin-walled region is fully suppressed. Thus, deformation or movement of the annular seal piece is fully suppressed. Consequently, even when a considerably great stacking load is imposed on the top panel wall of the container closure undergoing the dome phenomenon, sealing of the mouth-and-neck portion of the container by the container closure can be maintained fully reliably.
According to a second aspect of the present invention, as a plastic container closure for attaining the aforementioned technical object, there is provided a plastic container closure which is applied to a mouth-and-neck portion of a container, and which comprises:
a circular top panel wall; and
a cylindrical skirt wall extending downwardly from a peripheral edge of the top panel wall, and wherein
an annular seal piece, to be brought into intimate contact with the mouth-and-neck portion of the container, is integrally formed in an outer peripheral edge portion of an inner surface of the top panel wall, and an annular thin-walled region positioned radially inwardly of the annular seal piece is formed in the outer peripheral edge portion of the top panel wall.
Preferably, the annular thin-walled region has a width of 0.5 to 5.0 mm, and a thickness of 0.5 to 1.1 mm.